The present invention relates to hybrid blocked polyisocyanates, a process for their preparation their use as a crosslinking component for films and coatings, preferably for deep-drawable and corrosion-resistant stoving lacquers, in particular for the coil-coating process.
The formulation of blocked polyisocyanates with OH-containing polycondensates or polymers (polyesters or polyacrylates) to obtain binders for xe2x80x9cone-componentxe2x80x9d stoving lacquers is known. The utilization of xcex5-caprolactam, diisopropylamine (DIPA) and 1,2,4-triazole for blocking the isocyanate groups is also known.
It is usual today to deform uncoated metal sheets, assemble these to form a vehicle body and provide them with a primer in an electrophoretic bath.
Another possibility is to provide the metal sheets post-production with a suitable primer and store the coated coil temporarily. When the vehicle body is produced using the previously coated metal sheets, the coating must go through every deformation of the metal sheet undamaged.
An object of the present invention is to provide a coil coating having very good deformability and good corrosion protection.
It has now been found that the use of xcex5-caprolactam and either DIPA or 1,2,4-triazole as the blocking agent for the polyisocyanate component of the stoving lacquers provides coatings that have deep-drawability at room temperature and resistance to subsequent tearing after aging of the coatings.
The present invention relates to a blocked polyisocyanate containing
wherein the equivalent percents of isocyanate components A1) and A2) add up to 100%, and the equivalent percents of blocking agents B1) and B2) add up to 100 to 110%, based on the equivalents of component A).
The present invention also relates to a process for preparing these blocked polyisocyanates, to coating compositions containing these blocked polyisocyanates and to metal substrates coated with these coating compositions.
It is essential to the invention that a mixture of an NCO prepolymer having an elastifying effect (A1) and a highly branched lacquer polyisocyanate (A2) is used as the polyisocyanate component and a mixture of xcex5-caprolactam (B1) with either diisopropylamine or 1,2,4-triazole (B2) is used as the blocking agent, which dissociates at low temperature.
To prepare NCO prepolymer component A1), diisocyanates are reacted with dihydroxy compounds and, in small quantity, also trihydroxy compounds, in known manner. Preferably, 100 equivalent % of the diisocyanates are reacted with 40 to 45 equivalent % of the dihydroxy compounds and 5 to 10 equivalent % of the trihydroxy compounds.
Suitable diisocyanates include 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate or IPDI), bis-(4-isocyanatocyclohexyl) methane (H12 MDI or Desmodur(copyright) W, Bayer AG), 2,6- or 2,5-bis-isocyanatonorbornane, 1,4-bis-isocyanatomethyl cyclohexane and 1,3- or 1,4-tetramethylxylylene diisocyanate. Also suitable are aromatic diisocyanates such as 2,4- and 2,6-tolylene diisocyanate (TDI) and 4,4xe2x80x2- and 2,4xe2x80x2-diisocyanatodiphenyl methane (MDI). IPDI is preferred.
Suitable dihydroxy compounds the linear polyesters, polycarbonates and polyethers having a number average molecular weight of 500 to 3000, which are known from polyurethane chemistry. Preferred dihydroxy compounds are a mixture of adipic acid/neopentyl glycol/1,6-hexanediol polyesters having a number average molecular weight of 1700, in admixture with a polypropylene oxide polyether started on bisphenol A and having a molecular weight of 550. Additionally, low molecular weight diols having number average molecular weights of 62 to 400, such as neopentyl glycol or trimethyl pentanediol-1,3 may be used to raise the NCO content of the NCO prepolymer.
As indicated above, small quantities of trifunctional hydroxy compounds may also be used to improve the solvent resistance of the NCO prepolymer. Examples include trimethylolpropane or propylene oxide polyethers started on trimethylolpropane and having number average molecular weights of 250 to 1000.
The reactants are chosen such that the NCO prepolymers A1) have an NCO content of 5.0 to 10.0 wt. %.
Lacquer polyisocyanates A2) are known and are based on (cyclo)aliphatic diisocyanates having an NCO content of 12 to 25 wt. % and containing biuret, isocyanurate, allophanate, iminooxadiazine dione (asymmetrical trimer), urethane and/or uretdione groups. Examples of aliphatic and cycloaliphatic diisocyanates include 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate or IPDI), bis-(4-isocyanatocyclohexyl) methane (H12 MDI or Desmodur(copyright) W, Bayer AG), 2,6- and 2,5-bis-isocyanatonorbornane, 1,4-bis-isocyanatomethyl cyclohexane and 1,3- and 1,4-tetramethylxylylene diisocyanate. Polyisocyanates based on 1,6-diisocyanatohexane, IPDI and H12 MDI and containing predominantly isocyanurate groups are preferred.
Preferably NCO component A contains about 85 wt. % flexible NCO prepolymer component A1) and about 15 wt. % high functional lacquer polyisocyanate A2). Based on the NCO group content, this ratio is displaced in favor of lacquer polyisocyanate component A2). For example, if the total NCO content of component A) is 8.5%, NCO prepolymer A1) contributes only about 62%, while lacquer polyisocyanate A2) contributes about 38%.
xcex5-caprolactam (B1) and either 1,2,4-triazole (B2) or diisopropylamine (B2) are used as the blocking agents. The equivalent ratio of blocking agent B1 to blocking agent B2 is preferably from 0.3:1 to 0.7:1 to 0.5:1 to 0.5:1. Preferably, the equivalents of B1 and B2 adding up to 1.0 mole for each mole of NCO groups.
In a preferred embodiment for preparing the blocked polyisocyanates according to the invention an NCO prepolymer A1) is prepared first by adding an initial charge of the diisocyanate, for example IPDI, at room temperature and then adding the OH components, for example polyesters, polyethers, diols and triols, with stirring. The reaction of the NCO groups with the OH components is initiated by heating to 100 to 110xc2x0 C., and, when the calculated NCO content is reached or almost reached, the reaction is terminated. In the next step the NCO prepolymer is diluted with solvent (for example, solvent naphtha 100 solvent or 1-methoxypropyl acetate), and lacquer polyisocyanate A2), for example an HDI trimer, is added to form the total NCO component A1)+A2). The blocking reaction with xcex5-caprolactam (B1) is undertaken first at 100 to 110xc2x0 C. until the calculated NCO content is reached, and then the remaining NCO groups are is reacted with either 1,2,4-triazole or diisopropylamine.
The reaction with 1,2,4-triazole is endothermic and requires a reaction temperature of approx. 100xc2x0 C. To the contrary the reaction with diisopropylamine is exothermic, such that temperatures of only 40xc2x0 C. to 80xc2x0 C. are necessary to complete the reaction. When blocking of the NCO groups is complete, the blocked polyisocyanates according to the invention are ready for use and may also be diluted to working consistency with hydroxyl group-containing solvents, such as isobutanol or butyl glycol.
The hybrid blocked polyisocyanates according to the invention are polyhydroxyl compounds to form coating compositions that may be used to produce coatings on various substrates, such as wood, metal, glass, ceramic and plastics. They are preferably used to produce primers for coil coatings.
In addition to good corrosion resistance (salt spray test), the coil coating primers have flexibility and exceptional adhesion, which is manifested in the deep-drawability without damage of the coated metal sheets. Because the blocked polyisocyanates according to the invention are preferably aliphatic in character, i.e., they are prepared from aliphatic polyisocyanates, the primed coils may be stored before further processing, even outdoors, without the primer being broken down by light, which causes. By using the coated metal sheets obtained according to the present invention, cataphoretic coating can be omitted, for example in automotive construction.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.